High Density Plasma (HDP) oxide film is used to fill shallow trenches to isolate different transistor regions. The isolation HDP oxide filled regions are referred to as Field Oxide (FOX) for providing Shallow Trench Isolation (STI). To facilitate understanding, FIG. 1 is a flow chart of a prior art process of manufacturing on a semiconductor chip formed by at least one semiconductor device with a borderless contact using STI on a substrate. When the space between a contact edge and an active area edge is reduced to zero by design, these contacts are called borderless contacts and local interconnect. FIG. 2 is a schematic cross-sectional view of part of a substrate 204 of a semiconductor chip with field oxides 208 formed to provide Shallow Trench Isolation (STI). A nitride layer 212 is placed over the substrate 204 to provide an etch stop. An oxide layer 216 is placed between the nitride layer 212 and the substrate 204 to prevent damage to the substrate 204 by the nitride layer 212. Such protective oxide layers may be called “pad oxide layers.” Generally, a field oxide is deposited and then a Chemical Mechanical Polishing (CMP) is used to polish the surface so that excess FOX is removed, providing a flat surface as shown, using the nitride layer 212 as a stop for the polishing (step 104). The final height of the FOX may be >1000 Å above the substrate surface.
The nitride layer may then be etched away (step 108). Phosphoric acid may be used to remove the nitride layer. Such a wet etching may have a high selectivity between the nitride layer 212, the oxide layer 216, and the FOX 208. FIG. 3 is an illustration of the substrate in FIG. 2 after the nitride layer has been etch away. Due to the high selectivity, minimal amounts of oxide may be removed, providing negative angles 304 at the top of the FOX 208. A Hydrogen Fluoride (HF) wet etch may be used to remove the oxide layer 216, reduce the height of the FOX 208, and change the angle of the FOX 208 at a transistor active edge, by establishing a positive slope 404, as shown in FIG. 4 (step 112). The HF etch effectively moves the edges of the FOX back away from the transistor region.
A semiconductor device may be formed in the transistor region 408 between the FOXs 208 (step 116). An example of such a semiconductor device is illustrated in FIG. 5. In this example, the semiconductor device 504 comprises a source and a drain 508, a gate oxide 516, a polysilicon gate 520, and metal silicides 524 for forming electrical contacts. The process of forming the semiconductor device may expose the FOX 208 to as many as ten wet etches, which may etch away part of the FOX 208 to the point of the top of the FOX 208 that is in contact with the implant areas forming the source and drain 508. Such etchings may cause the edge between the FOX and the transistor to recess as much as 1100 Å.
A Nitride (SiN) layer 530 may be placed over the semiconductor device 504 and substrate 204, forming an etch stop barrier layer and spacers on the sides of the polysilicon gate 520 (step 120). An Inter-Level Dielectric (ILD) layer 540 may be placed over the nitride layer 530 (step 124).
An electrically conductive contact 604 may then be formed in the ILD layer 540, as shown in FIG. 6 (step 128). To form the contact 604, a trench may be etched in the ILD layer 540 and through the nitride layer 530. Such an etch may further etch the FOX 208.
To reduce Integrated Circuit (IC) sizes, current device designs may provide the placement of contacts that may be coincident with the edge of the transistor active region, so that part of the contact may be above the FOX, as shown in FIG. 6. If the FOX 208 has been etched too far, the contact 604 may come into contact with the well region of the substrate 204, which is the region of the substrate below the source or drain 508. Contact between the electrical contact 604 and the well region may cause a short to the well region, which may cause junction leakage.
The amount of FOX loss and recess may be enhanced with a PreGate Nitrogen Implant (PGNI), which may cause the FOX level to be below the transistor junction, causing a short to the well or ground. In addition, the variation in an ILD stack, consisting of both a layer of phosphorus oxide and silicon nitride, combined with plasma etch selectivity on both the type of the feature to be etched and the pattern density of features on a wafer, may result in longer etch time and a higher over etch percentage, to insure that all features are open to the transistor region. This higher percentage of contact (or LI Local Interconnect) over etch may punch deep into the field oxide, causing the final electrical connecting plug to make contact with the well region beneath the junction, causing a short to the wells. The variation of ILD thickness may also be dependent on polysilicon gate density. For ASIC devices, the polysilicon gate density may vary with different devices, making it hard to control ILD thickness, adding to the above-mentioned variation in an ILD stack.
In addition, there may be limitations on the selectivity between PSG and SiN during a PSG etch. There may also be limitations on the selectivity between SiN and FOX during an SiN etch.
It would be desirable to prevent contact between an electrical contact and the well region.